B3H7XH3 and H3BXH3 (X = N, P, and As) have been studied as donor-acceptor complex type at the G2-(MP2) level of theory. Both single- and double-bridged structures of B3H7 Lewis acid are taken into account. Although the double-bridged structure is energetically favored in the isolated state, the coordination prefers the single-bridged one. The monoborane adducts adopt the staggered arrangement with C-3nu, symmetry. The energetic analysis by natural bond orbital theory shows that the decrease of hyperconjugative contribution upon complexation in the B3H7 moiety has, as consequence, a loss of symmetry for B3H7XH3 (X = P and As) complexes. In the two series, the coordination is the result of two opposing interactions. The first one is stabilizing, developed between a' symmetry fragment molecular orbitals, and is mainly of 3MO-4e nature. The second one is destabilizing and developed between occupied molecular orbitals (2MO-4e) of donor and acceptor fragments having a" symmetry. Furthermore, in the case of BH3 complexes, the complexation is made along the C-3 axis that joined the two fragments, whereas for B3H7, it is mainly centered on the nonequivalent boron atom, but the two other ones are also weakly involved in the coordination through a suitable p orbital even if the donor fragment is not on the three boron triangle center.